The field of the disclosure relates generally to electric power systems, and, more particularly, to electric power distribution systems including transformers with tap changers and their methods of operation.
At least some known electric power systems include electric transformers configured to regulate voltages through the use of on-load tap changers. An on-load tap changing (OLTC) transformer has several connection points, so called “taps”, along at least one of its windings. With each of these tap positions a certain number of turns is selected. Since the output voltage of the OLTC transformer is determined by the turns ratio of the primary windings versus the secondary windings, the output voltage can be varied by selecting different taps. The tap position to connect to is determined by a suitable controller and tap selection is shifted through an on-load tap changing device. Since high voltages are involved, and the taps are changed while the OLTC transformer is under load, each time a tap is changed, arcing occurs. Arcing facilitates deterioration of the associated materials, thereby tending to decrease the service life of the tap changer mechanisms. Therefore, it is typically desirable to shift taps as infrequently as possible. However, it is not unusual to have dozens of tap changes over a 24-hour period, especially with an increasing share of variable and intermittent distributed generation (DG) and loads in the electric power system. The operators of the electric power system determine the tradeoff between the frequency and number of on-load tap changes with the subsequent wear on the tap changer and the quality of the voltage on the portion of the system maintained by the affected OLTC transformer.
Many known electric power systems include a growing share of distributed generation (DG), such as photovoltaic (PV) plants on residential rooftops, and loads, such as electric vehicles (EV), being connected to low voltage (LV) and medium voltage electric (MV) power networks and systems. As such, these additional DG and load points significantly increase the variability of the voltage on the portion of the system maintained by the affected OLTC distribution transformer, thereby increasing the frequency of commanded tap changes. In these cases, the critical voltage to be regulated (usually located at remote feeder ends) is spatially offset from the OLTC, which is located at the feeder head. Many remote feeder ends do not include voltage, current, and power measurement instruments due to the significant costs. Therefore, voltages at the remote feeder ends are typically controlled through regulating the voltage at the OLTC at the feeder head. Some known electric distribution systems have established, and in many cases, regulatory ranges for voltage regulation at the ends of the feeders, for example, within +/−5% or within +/−10% of established limits. As such, the voltage at the OLTC is regulated within a band tight enough to facilitate maintaining the feeder end voltage within established parameters, where the band needs to be sized to regulate the feeder end voltages without the aid of voltage instrumentation at the feeder ends.
Many known OLTC control systems are not configured to regulate the remote feeder voltages and maintain a lower number of tap changing operations for large amounts of DG and loads spread across the feeder. For example, some known OLTC control systems implement a variable bandwidth where the permissible voltage band at the OLTC is continuously adjusted based on the current network conditions as indicated by the measured power flow or current through the OLTC. However, this method assumes the worst case voltage drops and voltage rises for measured current or power flow in the associated feeders. For example, even at midnight, a worst case voltage rise of the largest PV power plant is assumed. These worst case assumptions limit the range of the variable voltage band at the OLTC, which may lead to unnecessary tap changing operations to facilitate maintaining the voltages at the remote feeder ends with a satisfactory margin to equipment parameters, i.e., some known electric distribution systems have ranges for the voltage at the end of the feeder within +/−5% or within +/−10% of the established limits.